PHP 二叉树递归遍历无限循环问题

Question

我有一个二叉树和节点class，可以创建节点，然后递归遍历根节点以获得前序、post 和中序节点顺序。这段代码在 JS 中有效，但由于某种原因无限循环并出现“不能在非对象上下文中使用‘$this’”的警告。在 addSide() 函数中返回 $this 时。是什么导致了这个无限循环，我该如何解决？

<?php

class Node {
      public $value;
      public $right = null;
      public $left = null;

      function __constructor($value) {
        $this->value = $value;
      }
    }

    class BinaryTree {
      public $root;

      function __constructor() {}

      function create($value) {
        $newNode = new Node($value);
        if (!$this->root) {
          $this->root = $newNode;
          return $this; //no warning
        }
        $current = $this->root;

        function addSide($side, $current, $newNode) {
          if (!$current->$side) {
            $current->$side = $newNode;
            return $this; //Warning: "Cannot use '$this' in non-object context."
          }
          $current = $current->$side;
        };
        while (true) {
          if ($value === $current->value) return $this;
          if ($value < $current->value) addSide("left", $current, $newNode);
          else addSide("right", $current, $newNode);
        }
      }

      function preOrder() {
        $visited = [];
        $current = $this->root;

        function traversePreOrder($node) {
          array_push($visited, $node->value);
          if ($node->left) traversePreOrder($node->left);
          if ($node->right) traversePreOrder($node->right);
        };
        traversePreOrder($current);
        return $visited;
      }

      function postOrder() {
        $visited = [];
        $current = $this->root;

        function traversePostOrder($node) {
          if ($node->left) traversePostOrder($node->left);
          if ($node->right) traversePostOrder($node->right);
          array_push($visited, $node->value);
        };

        traversePostOrder($current);
        return $visited;
      }

      function inOrder() {
        $visited = [];
        $current = $this->root;

        function traverseInOrder($node) {
          if ($node->left) traverseInOrder($node->left);
          array_push($visited, $node->value);
          if ($node->right) traverseInOrder($node->right);
        };

        traverseInOrder($current);
        return $visited;
      }
    }

    $tree = new BinaryTree();

    $tree->create(50);
    $tree->create(30);
    $tree->create(45);
    $tree->create(12);
    $tree->create(29);

    echo("inOrder: ". $tree->inOrder());
    echo("preOrder: ". $tree->preOrder());
    echo("postOrder: ". $tree->postOrder());

Answer 1

由于您似乎没有 PHP 背景，因此需要注意以下几点：

它是 __construct() 而不是 __constructor()。这成为值比较期间代码中的一个主要问题。
无需在函数内部创建函数。当一个方法被调用两次时，这可能会导致 cannot redeclare function 问题。
当从 class 中的另一个方法调用方法时，$this-> 是必需的，除非被调用的函数是 PHP 中的内置函数或至少在代码执行期间可用。
您似乎正在创建 二叉搜索树 而不仅仅是 二叉树.
在遍历过程中收集值时通过引用传递$visited。
您不能使用echo打印数组。使用 print_r() 或使用 implode() 使用定界符（比如 ,）将数组转换为字符串，然后使用 echo.
打印它
在create()中，你有时return一个节点，有时$this。两者不一样。前者是 Node class 的对象，后者是 BinaryTree class.
的对象
在create()方法中，只需要根据给定的值从当前代码开始向左或向右遍历即可，使用简单的while循环即可实现。

更正后的代码：

<?php

class Node {
  public $value;
  public $right = null;
  public $left = null;

  function __construct($value) {
    $this->value = $value;
  }
}

class BinaryTree {
  public $root;

  function __construct() {
    $this->root = null;
  }

  function create($value) {
    $newNode = new Node($value);
    if ($this->root === null) {
      $this->root = $newNode;
      return $newNode; //no warning
    }

    $current = $this->root;
    while($current !== null){        
        if($current->value > $value){
            if($current->left === null){
                $current->left = $newNode;
                break;
            }else{
                $current = $current->left;
            }
        }else if($current->value < $value){
            if($current->right === null){
                $current->right = $newNode;
                break;
            }else{
                $current = $current->right;
            }
        }else{
            throw new \Exception("Node with $value already exists.");
        }
    }
    
    return $newNode;
  }

  function preOrder() {
    $visited = [];
    $current = $this->root;
     $this->traversePreOrder($current,$visited);
    return $visited;
  }

  function traversePreOrder($node,&$visited) {
    array_push($visited, $node->value);
    if ($node->left !== null)  $this->traversePreOrder($node->left,$visited);
    if ($node->right !== null)  $this->traversePreOrder($node->right,$visited);
  }

  function postOrder() {
    $visited = [];
    $current = $this->root;
    $this->traversePostOrder($current,$visited);
    return $visited;
  }

  function traversePostOrder($node,&$visited) {
    if ($node->left !== null)  $this->traversePostOrder($node->left,$visited);
    if ($node->right !== null)  $this->traversePostOrder($node->right,$visited);
    array_push($visited, $node->value);
  }

  function inOrder() {
    $visited = [];
    $current = $this->root;
    $this->traverseInOrder($current,$visited);
    return $visited;
  }

  function traverseInOrder($node,&$visited) {
    if ($node->left != null)  $this->traverseInOrder($node->left,$visited);
    array_push($visited, $node->value);
    if ($node->right !== null)  $this->traverseInOrder($node->right,$visited);
  }
}

$tree = new BinaryTree();

$tree->create(50);
$tree->create(30);
$tree->create(45);
$tree->create(12);
$tree->create(29);

echo "inOrder: ". implode(",",$tree->inOrder()),PHP_EOL;
echo "preOrder: ". implode(",",$tree->preOrder()),PHP_EOL;
echo "postOrder: ". implode(",",$tree->postOrder()),PHP_EOL;

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